The present invention relates to porous materials, typically xerogels or aerogels, having a low dielectric constant but having a disadvantage of relatively poor mechanical strength. The present invention also relates to polymeric coatings, preferably parylene, coated on inorganic xerogels or aerogels so as to increase the mechanical strength while not substantially degrading the dielectric properties of the resulting coated material. Silica xerogel conformally coated with parylene AF-4 is described. An advantage of the present invention is to provide a low dielectric material having increased mechanical strength in comparison with the uncoated porous material, as would be advantageous in the fabrication of semiconductor electronic devices.
1. Technical Field
This invention relates generally to low dielectric constant porous materials and, more particularly, to porous materials conformally coated with polymeric materials for enhanced mechanical strength as well as low dielectric constant.
2. Description of Related Art
Porous materials have been used for a variety of applications including thermal insulators, heat storage systems, acoustic damping materials, and electrical insulators. In recent years, there has been particular interest in developing porous materials as insulators for use in semiconductor devices.
As feature sizes in integrated circuits are decreased to 0.18 xcexcm and below, problems related to signal delay, power consumption, and crosstalk become increasingly significant. Signal delay arises because the RC time constant of the materials becomes increasingly significant as interconnect linewidths and spacings have shrunk. Miniaturization also generally results in increased crosstalk or capacitive coupling between nearby conductors. One possible means for reducing the RC time constant and the capacitive coupling is to decrease the capacitance between the conductors. Since the capacitance between two conductors increases substantially in proportion to the dielectric constant of the medium separating them, this can be achieved by using a dielectric material with reduced dielectric constant separating the conductors.
Silicon dioxide (SiO2) has long been used in integrated circuits as the primary insulating material. With a dielectric constant of approximately 4, SiO2 has been an acceptable intermetal dielectric for many years. With the continuing shrinkage in film thickness and intermetal spacings, the SiO2 dielectric constant of 4.0 is now too high for optimum operation of high speed integrated circuits such as MPUs. One means for achieving a dielectric material with a lower dielectric constant than SiO2 is to use certain organic or polymeric materials. However, such materials tend to have limited thermal stability and mechanical strength. A number of materials are known to have dielectric constants around 2.6, such as polymers known commercially as SILK, FLARE and HOSP. Others such as parylenes have dielectric constants down to 2.24 (Parylene AF-4), and other variants are being tested which may have even lower dielectric constants. TEFLON, or related fluoropolymers are known to have dielectric constants down to approximately 1.7. However, because of their poor thermal and mechanical properties they are extremely hard to use in the manufacture of integrated circuits, where temperatures of 400 C. to 425 C. are normally encountered during subsequent processing steps. The dielectric constant of vacuum or dry air is about 1.0. Thus, one way to obtain a material with a low dielectric constant is to use a porous or sparse, low density material in which a significant fraction of the bulk volume consists of space or air. Other gases having low dielectric constant may also be acceptable. The effective dielectric constant will be determined by the combination of the dielectric constants of air and the porous material (or materials) and lie somewhere between the dielectric constants of air and the largest dielectric constant of the bulk material(s). Porous materials may be fabricated in many different structural forms with many different compositions. Therefore, such materials offer the possibility of achieving a low dielectric constant and having composition and/or structural features resulting in acceptable mechanical, thermal, electrical and chemical properties.
A low effective dielectric constant may be achieved by means of a porous structure in which a large fraction of the volume consists of space or low dielectric gas. However, it is not inherently necessary that such structures be open to the free flow of gas. Closed cell structures containing designed spaces containing vacuum or low dielectric constant gas, not open to free flow of gas throughout the structure, also achieve low dielectric constants.
One class of porous materials is foams. Typically, foams are made by producing a structure of the material and blowing air or other gases through the structure to create voids or by liberating gas throughout the material to create cells or pockets. Foams may be useful as thermal insulators and energy absorbers. However, the voids are typically on a macroscopic scale too large to be used in semiconductor devices with sub-micron characteristic feature sizes.
Another class of porous material that has been extensively investigated is sol-gel derived materials termed xerogels or aerogels. Sol-gel synthesis of oxide materials is typically based upon the hydrolysis and condensation of alkoxides M(OR)n where M is typically a metal atom (Si, Ti, Al, etc.) and R is typically an alkyl group. A common precursor for SiO2 xerogels and aerogels is tetraethoxysilane (xe2x80x9cTEOSxe2x80x9d), when M=Si, R=C2H5 and n=4.
Aerogels of main group oxides, and of transition and semimetal oxides, as well as carbon aerogels and aerogels of organic compounds, have been produced. Aerogels consist of molecular sized clusters which are connected in such a way that a three dimensional structure is formed resembling a microscopic xe2x80x9cstring of pearls.xe2x80x9d The rigid skeleton of the aerogel may occupy only a very small fraction of the total volume.
The most thoroughly studied xerogel system is probably silica, that is xerogels derived from silicon dioxide. As described, for example, in L. Hrubesh, Mat. Res. Soc. Symp. Proc. 381, p. 267 (1995) and in Fricke et al., Structure and Bonding 77, p. 37 (1992), silica xerogels are typically prepared by the controlled hydrolysis and condensation of a silicon alkoxide precursor such as tetraethoxysilane (TEOS), equivalently named tetraethylorthosilicate. A precursor solution of TEOS, alcohol, water, and an acid and/or base catalyst is typically mixed together. Hydrolysis and condensation first results in a xe2x80x9csolxe2x80x9d defined as a solution of polymeric or colloidal materials. The colloidal materials continue to agglomerate to form a xe2x80x9cgelxe2x80x9d. A gel is generally defined as a material composed of two phases, generally having a three-dimensional network of solid material with spaces within the network occupied by a liquid.
The next step in preparing a silica xerogel or aerogel is typically to dry the liquid or alcohol-filled gel without collapsing the structure. One method for drying the gel while avoiding the formation of a liquid-vapor meniscus is the method of supercritical extraction (also called xe2x80x9chypercritical extractionxe2x80x9d). This method avoids the creation of a liquid-vapor interface by the utilization of supercritical conditions of temperature and pressure during the solvent extraction process. Such conditions avoid surface tension forces and the collapse of the material due to the surface tension. Carbon dioxide is typically used for supercritical extraction since the critical point occurs at convenient values of temperature and pressure. However, the use of other solvents such as water is not inherently excluded. The material formed by supercritical extraction of the solvent is termed an aerogel.
Another technique for drying the gel while reducing surface tension forces makes use of evaporation, often preceded by a solvent exchange to achieve a reduction in surface tension forces during evaporation. One way to accomplish this is to perform a solvent exchange with a solvent that replaces the OH groups of the gel, but presents a hydrophobic surface when so bonded (or repulsive to the solvent if a non-aqueous solvent is employed). This reduces the solvent binding to the gel surface such that surface tension of the solvent does not substantially affect the structure of the gel during evaporation. Solvent exchange typically works best when employed in connection with thin film gels. The material produced in this way is generally termed a xerogel. The distinction between an aerogel and a xerogel lies in the processing method rather than in the final structure of the material. For economy of language we will use the term xe2x80x9cxerogelxe2x80x9d herein to refer to either xerogel or aerogel intending thereby that the present invention does not inherently exclude either procedure for forming the porous material. The present invention is independent of the formation processes leading to the particular porous material, xerogel or aerogel.
The potential utility of porous silica as a low dielectric constant insulating material has been recognized. (See, for example, Hrubesh (supra), Ramos et al. Mat. Res. Soc. Symp. Proc. 443, p. 91 (1997), and U.S. Pat. Nos. 5,470,802, 5,569,058, and 5,847,443.) However, as described by Ramos et al., (supra) for example, although porosity leads to a lower dielectric constant as compared with the corresponding dense material, certain disadvantages also arise. In particular, a tradeoff between dielectric constant and mechanical strength has been recognized. As a material is made more porous, the dielectric constant is decreased, but at the cost of a decrease in mechanical strength.
Another disadvantage of silica xerogels is their sensitivity to moisture. The inner surface of silica xerogels is very polar due to residual hydroxy or alkoxy groups bonded to silicon, which promotes adsorption of water. Chemical methods to modify this property by modifying the sol-gel synthesis method have been described by Schubert in Tailor-made Silicon-Oxygen Compounds, R. Corriu (Friedrich Vieweg and Sohn, 1996) pp. 263. However, this approach does not address the problem of mechanical fragility of porous materials. Another method to modify xerogel properties, using chemical vapor infiltration, in which a reactive gas is flowed through an xerogel placed in a furnace, has been described by Hunt et al., J. Non-cryst. Solids, 185, p.227 (1995). However, chemical vapor infiltration is described as resulting in new composite materials. These new composite materials may not retain the porosity of the original xerogel.
A way is needed to increase the mechanical strength of porous materials while retaining the advantageous low dielectric constant and other properties of these materials. Furthermore, for porous dielectric materials, it would be desirable if the method of increasing mechanical strength were readily integrated with standard semiconductor fabrication processes.
The approach taken in the present invention is to coat the porous material with a thin film of polymeric material to enhance mechanical strength while not unacceptably increasing the dielectric constant.